The ICRF44 monoclonal antibody reacts with human CD11b, a 165kDa adhesion molecule. CD11b associated with integrin beta2 (CD18) is expressed on the surface of monocytes, granulocytes, activated lymphocytes and a subset of NK cells. CD11b is a receptor for intercellular adhesion molecule family members CD54, CD102 and CD50 as well as for iC3b. These adhesions are crucial in cell-cell and cell-matrix interactions.